The deletrious of high oxygen concentration in the inspired air poses a serious clinical problem and its pathogenesis is not well defined. The proposed studies are designed to investigate the natural history of oxygen toxicity in both the ventilated and spontaneously breathing dog. These studies will correlate changes in the chest x-ray, lung mechanics, blood gas measurements, histology, phospholipid content and capillary filtration in the oxygen poisoned lung. Adultmongrel dogs will be anesthetized and ventilated or placed in a special chamber where they will be exposed to an envirnment of 100 percent oxygen at atmospheric pressures for 64-72 hours. Chest x-ray and blood gas mesurements will be correlated with measurement of lung mechanics to assess deterioration in lung function. Dogs will be allowed to become oxygen toxic (usually death occurs after 72 hours). Following the above mentioned measurements, lung weights will be done and microscopic sections taken as well as well as lung tissue obtained for study of phospholipid content. Serial measurements will be taken in both groups of animals to compare the deterioration of lung function so that the effect of ventilation on the natural history of oxygen toxicity can be studied. Following the initial study of two similar groups, lung biopsies will be taken for microscopic and phospholipid studies and the dogs removed from the oxygen environmnt and allowed to recover. They will be sacrificed at various inetrval and tissue obtained for assessment of lung recovery. Finally, to quantitate findings of increased lung weight and perivascular edema in the oxygen toxic lung, transcapillary fluid exchange will be studied to obtain an index of capillary filtration in both ventilated and spontaneously breathing dogs.